The present invention relates to aqueous chlorofluoropolymer dispersions. In particular, the present invention relates to aqueous chlorofluoropolymer dispersions having excellent long term ultraviolet light resistance, which can be coated onto a substrate at mild processing temperatures.
Polymers of chlorotrifluoro ethylene (CTFE) and copolymers of CTFE with vinylidene fluoride (VDF) have enjoyed significant commercial success both as a resin and a film. However, aqueous dispersions of these polymers have been of limited commercial value. Crystalline copolymers of CTFE/VDF, when the weight % of CTFE is significantly greater than VDF, regardless of how they are formed or processed, are very prone to embrittlement. Embrittlement is caused by exposure to elevated temperatures and/or ultraviolet radiation, which produce polymer morphological changes. Temperature induced embrittlement has precluded CTFE/VDF copolymers from many industrial applications servicing the chemical processing industry. Deterioration of mechanical properties by exposure to ultraviolet light has prevented CTFE/VDF copolymers from being used in outdoor applications.
For applications demanding a fluoropolymer, exceptional physical properties are required. Examples of these applications include aircraft textile coatings, stadium domes, industrial wall coverings, and awnings. For architectural stadium dome applications, for example, the following properties are required (Coatings Technology Handbook, xe2x80x9cArchitectural Fabricsxe2x80x9d (D. Satas ed., Marcel Dekker, Inc. New York, (1991) 717): (1) non-flammability is required to meet very demanding building codes, with a limited oxygen index (ASTM D2863) greater than 90% (the minimum percentage of oxygen in the atmosphere to support combustion of a material) being preferred; (2) resistance to smog and other environmental pollutants (the material 10 must be self-cleaning by exposure to rain water); (3) inert to all types of microorganisms; (4) UV resistant; (5) abrasion resistant; and (6) the composite should have a solar light transmission ASTM E424) adequate for the growth of grass.
Chlorotrifluoroethylene has been copolymerized with a large number of non-fluorinated olefins to produce coating compositions, both solvent borne and waterborne. These non-fluorinated olefins include as examples the vinyl ether copolymers disclosed by U.S. Pat. Nos. 5,200,480 and 5,304,617; the vinyl ester copolymers disclosed by U.S. Pat. No. 4,631,326 and the ethylene compolymers, disclosed by published Japanese Patent Application No. 208,969-1992. The use of significant levels of a non-fluorinated olefin will degrade many of these properties, particularly the non-flammability of the composition.
For many demanding applications requiring the use of a fluoropolymer, CTFE/VDF copolymers have many advantages over the perfluorinated polymers. In contrast to polytetrafluoroethylene which has poor abrasion resistance and low solar light transmission, polymers of CTFE and CTFE/VDF are abrasion resistant and when formed into a film have  greater than 90% solar light transmission. CTFE/VDF copolymers have many fabricating advantages over PTFE, polytetrafluoroethylene-hexafluoropropylene copolymers (FEP) and polytetrafluoroethylene-hexafluoropropylene-vinylidenefluoride (THV, manufactured by Dyneon) terpolymers. CTFE/VDF copolymers can be processed at mild temperatures and can be easily recoated by successive coatings of aqueous CTFE/VDF copolymer dispersions. Because CTFE/VDF copolymers have a higher surface tension than PTFE, FEP, and THV, recoating with successive passes of CTFE/VDF copolymers can be accomplished with very low levels of wetting agents, 0.1-2 weight % being typical depending on whether the wetting agent additive is fluorinated, perfluorinated, or non fluorinated, or any mixture thereof. PTFE, for example, is typically processed after adding 5-10 weight % of a non-ionic wetting agent from 300-375xc2x0 C., which releases large quantities of decomposed surfactant into the oven coating towers. CTFE/VDF copolymers are excellent film formers which facilitates the production of dispersion cast film for lamination onto fabric, or the direct coating/recoating of fabric to a desired thickness with low levels of wetting agents.
CTFE/VDF copolymers are disclosed by McCarthy et al., Proceedings of the Twenty-Fifth International Water-Borne, High Solids and Powder Coatings Symposium 541 Feb. 18-20, (1998) and Bringer, Encyclopedia of Polymer Science and Technology (1st ed., vol. 7, Interscience Publishers, New York, 1967) p. 204, and by International Patent Publication Nos. WO 97/11979 and WO 97/17381. However, polymers of CTFE and CTFE/VDF as described by the prior art do not satisfy all of the criterions of a stadium dome textile coating/laminate, particularly UV resistance.
Prior art fabric coatings used in outdoor applications consisted of polyvinyl chloride, acrylics, PVDF, PVF, polyurethane, and rubbers such as neoprene. These materials suffered from being either too flammable, UV sensitive, susceptible to morphological changes, poorly adhesive to substrates (which is a current problem for PVF in the manufacture of awnings), or in some cases not sufficiently translucent.
For very demanding applications in which the composite is required to be load bearing, PTFE-coated fiberglass emerged in the 1970s as the material of choice. In load bearing structures such as stadium domes, wherein the coated glass fabric must meet the previously described requirements, it must above all perform as a material of construction. Load bearing structures such as stadium domes must survive the handling encountered during construction, as well as installation loads, prestress loads, and live loads including wind shear. The structures are prestressed to a certain tension and elongation, so that they are more resistant to further stresses.
These types of architectural fabrics are typically designed so that the maximum load the fabric will encounter once installed is no greater than 25% of the maximum tensile breaking strength of the fabric. For these reasons, it is very important that the polymer used in making the composite, whether it is laminated onto the glass fabric or impregnated into the fabric, must show little or no change in its mechanical or physical properties over the course of the thirty to fifty years the fabric might be in place. PTFE has satisfied these requirements, but has a large drawback because it produces composites with poor solar light transmission that do not support the growth of natural grass, requiring the use of artificial turf.
Copolymers of CTFE and VDF are disclosed by McCarthy et al., Proc. 25th, Inter. Waterborne, High Solids and Powder Coat. Symp., 541 (Feb. 18-20, 1998). However, CTFE/VDF copolymers having significant levels of crystallinity do not satisfy these requirements. There remains a need for polymer compositions that satisfy the previously described physical properties.